Trocar

For a long time there was a persistent fan rumour that Snape might be a vampire. While it is true that he has an unhealthy pallor, and is sometimes described as looking like a large bat in his long black cloak, he never actually turns into a bat, we meet him outside the castle by daylight, and no corpses with puncture marks in their necks ever turn up at Hogwarts. In short, Snape is not a revamped Trocar.
-- J.K. Rowling (Pm)

J.K. Rowling had notes about a possible vampire teacher at Hogwarts named “Trocar” but the character was discarded fairly early in the writing process. He did not evolve into Snape, in spite of popular fan theories about the Potions Master’s “bat-like” appearance and “unhealthy pallor” (Pm).